The aging central nervous system (CNS) is characterized by an elevation in baseline inflammation. This contributes to the pathogenesis of many CNS diseases. A significant portion of this inflammation arises from microglia, the resident innate immune cell of the CNS. Compared to microglia of young mice, aged microglia shift toward a pro-inflammatory phenotype and secrete more damaging pro-inflammatory cytokines. However, the mechanism by which microglia become more pro-inflammatory with age is unknown. Recent studies and preliminary data from our lab demonstrate the importance of epigenetic regulation in microglia polarization. Our preliminary data show that inhibition of Enhancer of Zeste Homologue 2 (Ezh2) decreases pro- inflammatory M1 and rescues anti-inflammatory M2 polarization. This suggests Ezh2 function favors pro- inflammatory M1 microglia polarization. We hypothesized that higher levels of Ezh2 may be responsible for the pro-inflammatory phenotype of aged microglia. Ezh2 levels were assessed in young and aged mice. Consistent with our hypothesis, we found higher levels of Ezh2 in the aged brain. Thus, Ezh2 may be responsible for the enhanced pro-inflammatory status of microglia in the aged brain. In this proposal, we aim to determine if Ezh2 simultaneously promotes M1 and suppresses M2 microglia polarization. We will use primary neonatal murine microglia cultures to elucidate the molecular mechanism by which Ezh2 promotes M1 and represses M2 polarization. Additionally, we aim to determine if peripheral factors contribute to the up- regulation of Ezh2 in the central nervous system with age. We will use heterochronic parabiosis to examine the role of peripheral factors on microglia phenotypes with age. We will also age microglia-specific Ezh2 knockout mice and assess microglia phenotype at baseline and evaluate their functionality relative to wildtype aged controls by assessing their response following an acute ischemic injury. When completed, the proposed work will provide novel insight into how microglia transition to a primed pro-inflammatory phenotype as they age. This is essential to our understanding of the pathogenesis of CNS diseases. This work will also provide the PI with the necessary training to become a proliferative future physician-scientist.